1. Field of the Invention
This invention relates to electrical double-layer capacitors. Particularly, this invention relates to low temperature electrical double-layer capacitors using asymmetric and/or spiro-type quaternary ammonium salts.
2. Description of the Related Art
The storage of electrical energy and delivery of electrical power at low temperatures is difficult to achieve. In part, this is because most typical energy storage technologies, such as chemical batteries, utilize electrochemical processes which are highly temperature dependent and cannot provide high currents at lower temperatures. Thus, chemical batteries in cold environments must typically be maintained at a temperature higher than the ambient environment (requiring additional mass and/or power dedicated to thermal management). Alternately, a chemical battery may be oversized to compensate for the compromised low temperature power density. In addition, other power sources such as radioisotope thermoelectric generators cannot provide high power levels. Applications involving extremely cold environments, e.g., space and Arctic conditions, require solutions to deliver adequate electrical power while mitigating the weaknesses of conventional solutions, e.g., additional mass and/or system complexity.
Such double-layer capacitors (also known as supercapacitors or electrochemical capacitors) combine the high power density (e.g., approximately 1,000 W/kg) provided by electrolytic capacitors with a moderate energy density (e.g., approximately 5-10 Wh/kg), enabling very high current pulses to be delivered for short bursts of time, or alternatively a very low current for extended periods of operation. Due to the unique mechanism of charge storage (i.e., the double layer formed at a solid/liquid interface), cycle life is nearly infinite (i.e., greater than 106 cycles). Despite these favorable attributes, commercially available components employing non-aqueous electrolytes (e.g., from Maxwell Technologies or Cooper Bussman, two leading suppliers of double-layer capacitors) are generally limited to operation at temperatures greater than −40° C. due to the relatively high freezing point of the standard electrolytes used. Operation to at least −55° C. (where most space-rated avionics are required to operate) is necessary to allow these components to be easily integrated with existing space avionics.
FIG. 1 is a schematic diagram of a conventional double-layer capacitor 100. The double-layer capacitor 100 comprises two electrical layers 102A, 102B. Each layer 102A, 102B includes a porous electrode 106A, 106B, typically carbon. Note that each porous electrode 102A, 102B in FIG. 1 appears as multiple separate particles, however, the particles are interconnected in a “sponge-like” structure as will be understood by those skilled in the art. A chemical electrolyte solution 108 fills each layer in the interstices of the porous electrodes 106A, 106B. The chemical electrolyte solution 108 typically comprises a salt, such as tetraethylammonium tetrafluoroborate (TEATFB), dissolved in propylene carbonate or acetonitrile. The layers 102A, 102B are isolated from each other by a separator 110 that is both ionically conducting and electrically insulating to the electrolyte solution 108. A voltage differential across opposite ends of each layer 102A, 102B (from conductive contacts to each porous electrode 106A, 106B) induces a positive charge in one porous electrode 106A and a negative charge in the other porous electrode 106B which attract negative ions 104A and positive ions 104B of the electrolytes solution 108 to each electrode 106A, 106B, respectively. In each layer, energy storage is obtained by the charge separation between respective ions 104A, 104B in the electrolyte solution 108 and the surfaces of the porous electrodes 106A, 106B. The porous structure of the electrodes 106A, 106B, providing a very high effective surface area (e.g., greater than 1000 m2/g), coupled with the extremely short (molecular level) effective charge separation between the ions and those surfaces, yields a high energy density for the double-layer capacitor 100.
Due to their high power density, simple construction and near infinite cycle life, double-layer capacitors have been considered for applications where very high current pulses need to be delivered in extreme environments, such as under high G-loading or in high radiation conditions. See, Conway, “Electrochemical Double-layer capacitors: Scientific Fundamentals and Technological Applications,” New York: Kluwer-Plenum, 1999; Merryman et al., “Chemical double-layer capacitor power source for electromechanical thrust vector control actuator,” J. Propulsion Power, vol. 12, 89-94 (1996); and Shojah-Ardalan et al., “Susceptibility of ultracapacitors to proton and gamma irradiation,” 2003 IEEE Radiation Effects Data Workshop Conference Proceedings, 89-91 (2003).
Despite the relatively rugged nature of double-layer capacitors, wide temperature operation is usually not possible, with −40° C. representing the typical lower rated limit for commercially available non-aqueous off-the-shelf parts. However, the need for energy storage and power delivery technologies that can operate at low temperatures is of great interest for space exploration applications. This need has led to the successful development of low temperature lithium ion batteries in recent years. See, Ratnakumar et al., “Lithium batteries for aerospace applications: 2003 Mars Exploration Rover,” J. Power Sources, vol. 119-121, 906-910 (2003). In order for currently available double layer capacitors to find use in space avionics, they would require special thermal control apart from the rest of the electronic subsystems, since most space rated electronics can operate to at least the −55° C. limit. Aqueous double-layer capacitors which can operate to −55° C. are available, however, the theoretical energy density of these components is a factor of five lower than non-aqueous cells, due to the lower operating voltage.
Some double-layer capacitors have been characterized extensively down to −40° C., mainly to study fundamental electrode processes and characterize leakage phenomena. See, Gualous et al., “Experimental study of double-layer capacitor serial resistance and capacitance variations with temperature,” J. Power Sources, vol. 123, 86-93 (2003); Kotz et al., “Temperature behavior and impedance fundamentals of double-layer capacitors,” J. Power Sources, vol. 154, 550-555 (2006); and Janes et al., “Use of organic esters as co-solvents for electrical double layer capacitors with low temperature performance,” J. Electroanal. Chem., vol. 588, 285-295 (2006) (Janes). These data indicate that down to −40° C., the performance is acceptable for a range of applications. However, there is a dearth of data for double-layer capacitors below this temperature limit, limited by the high melting point of the solvents used in commercially available cells (typically, propylene carbonate or acetonitrile). Double-layer electrochemical capacitors may be potentially attractive to operate at low temperature, because diffusion of ions occurs over very short distances, unlike batteries. The most significant challenges in designing double-layers capacitors for low temperature operation are to suppress the melting point of the electrolyte to below the desired operating temperature, to maintain solubility of the salt in the electrolyte solution at low temperatures, and to minimize increases in equivalent series resistance due to increased solvent viscosity accompanying the reduced temperatures.
In view of the foregoing, there is a need in the art for apparatuses and methods of producing double-layer capacitors capable of operating at extremely low temperatures, e.g. at or below −40°. There is also a need for such apparatuses and methods employing electrolytes that avoid freezing, maintain salt solubility and mitigate the increase in equivalent series resistance (ESR) due to increases in solvent viscosity at low temperature. There is particularly a need for such apparatuses and methods in space applications to eliminate the need for additional system complexity, e.g., special thermal control, and without requiring additional mass for the energy storage. These and other needs are met by the present invention as detailed hereafter.